Frequency-division velocity modeling technology and its application in igneous rock development area: a case study of G structure in Xihu Sag

The G structure in the Xihu Sag of the East China Sea Shelf Basin is influenced by tectonic inversion and volcanic activities, leading to significant lateral velocity variations. The frequency information of the seismic velocity field obtained by traditional processing method is limited, and it is difficult to reflect the detailed information of igneous rocks of different scales, making it challenging to predict accurate structural feature. To solve the problem, a new method for frequency-division reconstruction of the seismic velocity field and a hybrid interval velocity model for time-depth conversion, which is applicable to areas with structural inversion and the development of igneous rocks, is proposed: First, igneous rocks are identified and classified by scale through multi-attribute analysis. For large-scale igneous rocks and lateral velocity trends, low-frequency velocity components are derived via high-resolution nonlinear grid tomography combined with pre-stack depth migration, further enhanced by anisotropic velocity modeling to improve complex structural imaging. For small-scale igneous rocks, high-frequency velocity components are obtained through pre-stack simultaneous inversion. The high- and low-frequency velocities are then integrated to reconstruct a seismic interval velocity field, which is not only consistent with the structural trend, but also can reflect the local details. Finally, under structural model constraints, well-derived velocities are used to calibrate the frequency-division velocity model, yielding a hybrid interval velocity model for time-depth conversion. Based on this method, it is recognized that there is a migration of structural high in the deep and shallow layers of G anticline, which supports the drilling of an exploration well, with pre-drill and post-drill structural interpretations showing high consistency and depth prediction errors within 10 meters, validating the method's reliability. Inspired by the new understanding of structure G, it is found that the structural high migration of other anticlinal structures in Xihu Sag is universal through variable velocity mapping. It is pointed out that the early high points in the deep layer are more conducive to the early convergence and accumulation of oil and gas, which are favorable exploration direction for the next stage.